1. Field of the Invention
The present invention relates to a reconfigurable device for amplifying and combining RF (radio frequency) signals.
In particular, although not exclusively, it applies to satellite transmission devices using transmission frequencies in the microwave band, for example on the order of 20 GHz or above, or in Ka band (approximately 35 GHz).
2. Description of the Related Art
Conventionally, devices of this kind comprise a passive or active power divider network to which the input signal is applied, for distributing the input signal power among several power signals that are amplified by means of a respective amplifier, for example of the SSPA type (Solid State Power Amplifier).
Thereafter, the various amplified signals are applied to a switched power combiner circuit based on delay lines, providing a signal whose power is the sum of the powers of the amplified signals, wherein the desired output RF power determines the number of amplified signals to be combined, which is selected by means of switches. The combiner circuit output signal is input to a switched impedance transformer circuit which supplies an amplified output signal. All of these circuits comprise electromechanical or electronic switch units for activating the desired number of amplifiers and for carrying out the power summation function.
According to the frequency, the RF power level, the number of input branches and the required combining or routing flexibility, this device may be quite bulky and heavy, highly complex, both on an electrical and a mechanical point of view. Furthermore, insertion losses, specifically in the switch units, may be very large, even in the DC component of the signals, and the switch units consume a large amount of power.
Moreover, these devices have a limited reliability which, in particular, results in the use of vacuum tube amplifiers and electromechanical switch units.
Due to these drawbacks, such a device is not well adapted to be embarked on a satellite.
In addition, there are microwave amplifiers implemented as integrated circuits. However, these amplifiers have insufficient output power, specifically for transmission systems used in communications satellites. This drawback can be overcome by using several amplifiers in parallel preceded by a power divider circuit to which the signal to be amplified is input and followed by a power combining circuit which supplies a signal having a power equal to the sum of the powers output by the respective amplifiers.
Such devices are, for example, known from U.S. Pat. Nos. 4,315,322, 4,780,685 and 4,965,530. However, when one of the amplifiers fails, it affects the operation of the remaining circuitry by causing power losses due to impedance mismatch. It is therefore necessary to provide means for monitoring the operation of each amplifier and for disconnecting a faulty amplifier by grounding the corresponding amplifying channel by means of switches. The presence of switches in the off position also proves to cause power losses and a circuit mismatch. As a consequence, when the number of switches in the off position increases, losses increase. Accordingly, it is not desirable that one or several amplifiers be thus disconnected.
It is an object of the present invention to eliminate these drawbacks and to provide a device of the above-described type wherein the output power may be adapted on demand, which device comprises a power divider network to which the input signal is applied and which distributes the signal among several channels, each of the channels comprising an amplifier, the amplifier outputs being applied to a power combiner circuit comprising a quarter wave impedance transformer circuit for each channel connected to the device output.
According to the present invention, the amplifier of each channel comprises an integrated circuit including a controllable output transistor for deactivating the amplifier, the power combiner circuit comprising a quarter wave impedance transformer circuit for each channel connected to a common node coupled to an output of the device.
With respect to the prior art devices, the present invention not only allows lighter, less bulky and more reliable amplifiers to be used, but also to eliminate a switch and a quarter wave delay line per channel. This results in a circuit which is much less bulky and lighter and which requires reduced supply power. This solution also has the advantage of leading to a greater reliability as a result of its redundant construction wherein the amplifiers operate in parallel, and a defective amplifier can be replaced by an idle amplifier.
Advantageously, semiconductor power amplifiers are composed of solid-state monolithic integrated circuits and are controllable through their drain current, so that the prior art switched impedance transformer circuit can further be eliminated.
In such a device, the quarter wave impedance transformer circuits in the power combiner circuit determine the load of the power amplifiers and therefore, the gain of the latter, which increases if the number of activated amplifiers is reduced. As a result, the transistors in the amplifiers reach saturation more rapidly, so that the power handling capability of the device decreases.
According to a feature of the present invention, this drawback is overcome by a load impedance control circuit arranged at the output of the power combiner, and constituted by a quarter wave impedance transformer circuit with adjustable impedance.
According to another feature of the present invention, this device comprises means for adjusting the bias level of the amplifiers as a function of the number of activated amplifiers. In particular, the increase in the load impedance due to a decrease in the number of activated amplifiers is compensated for by a decrease in the bias of the amplifier drain current. This solution has the advantage of requiring a reduced number of switch units, since the same drain current may be applied to all amplifiers.
A combination of the latter two provisions may also be envisioned, for example, by using a simplified load impedance control circuit and two levels of amplifier drain current.
An embodiment of the device according to the present invention will be described below by way of non-limiting example with reference to the attached drawings.